There are more popular Mohels and less popular Mohels. A popular Mohel can sometimes do three Brissim a day, whereas a less popular Mohel may be lucky to do one every two weeks.
From my own experience with my children, the more popular Mohels will suggest a lower amount, in the $300-$500 range, whereas the less popular ones will suggest more. Some Mohels will suggest an amount based on your financial standing.
This may be different in your locale, but this is the way it is where I live.